Hepatitis G virus (HGV) is a recently identified RNA virus which appears to be transmitted by parenteral exposure to blood and blood products, and may be associated with clinical hepatitis in humans. The objective of this study was to evaluate the prevalence of HGV in HCV infected individuals and in HCV contaminated Immune Globuin Intravenous (IGIV) products. Different sets of nested primers, specific for the 5' non-coding region of HGV, were designed and tested in a reverse transcription and polymerase chain reaction (RT-PCR) assay. The set of primers that was found to be most effective in detecting HGV was used to test serum samples from chronically infected HCV positive (HCV 2.0 ELISA, RIBA 2.0 and HCV RNA positive) patients and 6 HCV RNA positive IGIV lots. HGV specificity of the PCR products was confirmed by sequencing and comparing them with the published sequence for HGV. HGV RNA was detected in 14 of the 50 (28%) HCV infected patient serum samples, and in 2 of the 6 (33%) IGIV lots. These results indicate that HGV RNA is quite prevalent among HCV positive patients and is also present in some HCV contaminated IGIV lots. The clinical significane of HCV and HGV co-infection needs to be evaluated. Moreover, since the manufacture of IGIV now includes steps to inactivate or remove certain lipid enveloped virus, it is not known whether these processes also act similarly on HGV.